1. Field of the Invention
The present invention relates to an acoustic system comprising a sound collecting unit and a sound output unit.
2. Description of the Related Art
As a sound masking device (method), there has been conventionally known the one disclosed in Patent Document 1 (the title of the invention: “SOUND MASKING SYSTEM, AND METHOD AND PROGRAM FOR GENERATING MASKING SOUND”) or the one disclosed in Patent Document 2 (the title of the invention: “DEVICE FOR PROTECTION OF SPEECH PRIVACY”), for example. Patent Document 1 only describes using a microphone as a means for picking up sounds in a conversation between speaking persons. Patent Document 2 discloses a speech privacy device.
In a sound transmission path including a microphone and a loudspeaker, when the microphone and loudspeaker are placed in the same space, they certainly form a loop and may sometimes cause acoustic feedback or an echo. Acoustic feedback occurs when a microphone picks up sound from a loudspeaker besides voice of a speaking person or a vocalist. The sound received by the microphone from the loudspeaker is amplified by an amplifier, and further amplified by the loudspeaker. Thereafter, the louder sound is received by the microphone again, amplified by the amplifier, and further amplified by the loudspeaker, causing so-called positive feedback. Such repetitions, i.e., a loop state among the microphone, amplifier, and loudspeaker, will cause a sound recognized as a screech or a boom.
Patent Document 1 states that, when a microphone 30 and a loudspeaker 40 are provided in an acoustic space 20A, a masking sound is generated based on a conversation between users present in the acoustic space 20A, and the generated masking sound is audibly produced in the same acoustic space 20A, so that both the conversation and the masking sound can be overheard in an acoustic space 20B. As a result, it is difficult for users present in the acoustic space 20B to understand the content of the conversation between the users present in the acoustic space 20A. However, in this case, acoustic feedback could occur because the microphone 30 and loudspeaker 40 are provided in the same acoustic space 20A. In this regard, Patent Document 1 proposes that the microphone 30 and loudspeaker 40 be appropriately positioned and appropriate signal processing be performed so that acoustic feedback will not occur.    [Patent Document 1] Japanese Patent Application Laid-open No. 2008-233671    [Patent Document 2] Japanese Patent Application Laid-open No. 2006-267174    [Non-Patent Document 1] F. Kawakami and Y. Shimizu, “Active Field Control in auditoria”, Appl. Acoust., 1990, 31, p. 45-47
Patent Document 1 suggests that acoustic feedback be prevented when the microphone 30 and loudspeaker 40 are provided in the same acoustic space. However, such a suggestion sometimes does not suffice to prevent acoustic feedback or echoes. That is all because the aim of the system in Patent Document 1 is positive use of feedback loop by smoothing the frequency response between loudspeakers and microphones.